Jupiterimages/Thinkstock(NEW YORK) -- Women in the military have access to some of the nation's best health care, which includes free birth control. But a new study shows that many women are not using it and the rate of unintended pregnancy is double that of the general population.

And today, with the Department of Defense ending its longtime ban on women serving in combat roles, an unplanned pregnancy could have wider ramifications not only for a woman's health, but for her opportunities for advancement.

An estimated 10.5 percent of active duty women, ages 18 to 44, reported an unplanned pregnancy in the prior 12 months in 2008, the last year for which there are statistics, according to researchers at Ibis Reproductive Health, a nonprofit organization that supports women's sexual and reproductive rights.

That number was higher than in 2005, when the rate was 9.7 percent.

In the non-military population, about 5.2 percent of women of reproductive age report an unintended pregnancy each year, according to the study, published this week in the February issue of Obstetrics and Gynecology.

The Ibis study was based on surveys of more than 7,000 active-duty women; the statistics were obtained from the Department of Defense under the Freedom of Information Act. Rates were equal among those women who were deployed and those serving stateside.

Women make up 202,400 of the U.S. military's 1.4 million active duty personnel; more than 280,000 women have deployed over the last decade to Iraq and Afghanistan.

"It's terrific that women are getting recognition for their role in combat missions and are being considered for all types of promotions in the armed services," said lead author Kate Grindlay, senior project manager at Ibis. "But for women to reach their potential, they must be able to access birth control for their personal health and well-being."

About 900 women had been unable to deploy in the past year due to a pregnancy, either planned or unplanned, according to the study. The highest rates were among younger women with less education who were either married or cohabitating, researchers said.

The authors of the study say that an unwanted pregnancy not only disrupts a woman's military career, but takes a toll on military readiness because pregnant women cannot be deployed or must be evacuated from war zones. They say the military needs to take a more "comprehensive approach" to address the problem.